SheepShaver won't compile correctly in Xcode 8 I think, but your current issue is that you haven't installed the SDL development libraries. Once you install those from sdl.org, THEN you'll get the compiler errors regarding LLVM.

First you'll need to get Snow Leopard up and running inside a virtual machine, and then you'll need to use that as your build environment so you can use gcc 4, which is the last version of gcc (or llvm) that will build the insecure code structure SheepShaver requires.

I followed the instructions and the Debug built fine, but the Release did not. It complained that it couldn't find the SDL framework, but I d'led the SDL framework from the provided link and copied it into /Library/Frameworks as instructed.

When I ran the debug version, it immediately quit and Xcode said it exited normally. Is that "normal?"